Juul Labs is under fire again. But this time, the US Food and Drug Administration (FDA) has declared youth vaping an "epidemic" and started an unprecedented crackdown on retailers and e-cigarette manufacturers. FDA Commissioner Scott Gottlieb announced the new series of actions, and threatened a potential ban on flavored e-cigs, in an open letter on Wednesday.

After an "undercover, nationwide blitz" this summer against online and brick-and-mortar retailers who sell e-cigarettes, the agency has issued more than 1,300 warning letters, some including hefty fines, to retailers who illegally sold the Juul and other e-cigarettes to minors. But it doesn't stop there.

In the letter, the FDA warned that Juul Labs and the makers of four other popular e-cigarettes (Blu, Logic, Vuse, MarkTen XL) have 60 days to submit plans that prove they can keep their products out of the hands of minors. If they fail to do so, the FDA said it may require brands to remove "some or all of their flavored products" from the market.

According to the FDA, this is the largest coordinated enforcement effort in the agency's history — the latest in a series of actions that are part of the FDA's Youth Tobacco Prevention Plan, aimed at preventing youth access and reducing tobacco-related disease and death.

"While we remain committed to advancing policies that promote the potential of e-cigarettes to help adult smokers move away from combustible cigarettes, that work can’t come at the expense of kids. We cannot allow a whole new generation to become addicted to nicotine," Gottlieb said in a statement.

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There are many different e-cigarettes, also called electronic nicotine delivery systems (ENDS), on the market. But the Juul — often dubbed the "iPhone of E-cigs" — has gained unparalleled popularity in the last year.

The battery-powered device works by converting liquid into a vapor, which the user inhales. The liquid is inside pre-filled cartridges called "JUULpods," which come in flavors like fruit medley and crème brûlée at 5% or 3% nicotine strength. Pods contain about as much nicotine as a pack of cigarettes and are meant to give 200 puffs. The devices are small, discrete, and easy to use in places where smoking is unacceptable.

Those features appeal to adult smokers who want to switch from conventional cigarettes, but also make it easy for young people to use them without being detected by parents or teachers. The long-term health effects of e-cigarettes like the Juul are not yet known, but nicotine is a highly addictive substance that can affect brain chemistry and development in young users, who may not even be aware they are consuming nicotine.

In April, the FDA sent warnings to 40 retailers selling the JUUL to minors, and asked the company to provide information about their marketing strategies and research on the physiological and behavioral effects of the Juul. Several Juul users or their parents have filed lawsuits this year against the manufacturer, alleging that the e-cigarette has worsened their nicotine addiction.

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According to a statement from Juul Labs CEO Kevin Burns, the company will work with the FDA in response to its requests and they said they are committed to preventing underage use of the product. "Our mission is to improve the lives of adult smokers by providing them with a true alternative to combustible cigarettes," Burns said.

However, this FDA crackdown on both the companies making these products and the retailers selling them to minors could have major implications for the multibillion-dollar e-cigarette industry.

"In enabling a path for e-cigarettes to offer a potentially lower risk alternative for adult smokers, we won’t allow the current trends in youth access and use to continue, even if it means putting limits in place that reduce adult uptake of these products," said Gottlieb.

Caroline Kee is a health reporter for BuzzFeed News and is based in New York.